The present invention is related to low oxidation power, i.e. at low temperature and low oxygen concentration, thermal oxidation processes in the presence of a chlorine source. Said processes can be used during the manufacturing of semiconductor devices. Specific examples of use of such processes are the growth of ultra-thin oxide layers, the Cl-cleaning of a substrate and the temperature ramp-up cycles prior to the oxide growth.
In thermal oxidation processes the aim is to grow SiO2 films by exposing silicon to O2 at elevated temperatures. Historically chlorine has been introduced in the oxidation ambient in order to improve the electronic quality of gate oxide layers. Studies have revealed that the improvements by introducing chlorine are in fact initiated by the presence of Cl2. Particularly the reduction of electronic instabilities, attributed to the presence of mobile ions mainly Na, has been emphasized. In addition, the use of Cl during gate oxidation was also found to result in a reduction of the density of dielectric breakdown defects and of stacking faults. It has been demonstrated that metal contamination on the wafer surface prior to gate oxidation has a distinct negative effect on the dielectric integrity of thin oxides. Particularly Ca has been identified as one of the most detrimental metals in that respect. The introduction of Cl in the oxidation ambient was found to be very efficient in removing metal contaminants, especially Ca, from the silicon wafer surface. In order to meet the stringent future gate-oxide defect density requirements, the residual concentration of metals and of Ca in particular has to be further reduced.
Most oxidation tools are now equipped for the introduction of chlorine species during silicon wafer oxidation and/or in situ tube cleaning operations. Several sources have been used to introduce chlorine. In order to compare these different methods a common parameter describing the concentration of the total amount of Cl fed to the reactor chamber, irrespective of its chemical state, is introduced. Said parameter is the xe2x80x9cchlorine-equivalent concentration of a given Cl-sourcexe2x80x9d and is defined as the ratio between xe2x80x9cthe total flow of Cl atoms [number of Cl atoms per unit time ] to the process chamberxe2x80x9d and xe2x80x9cthe total flow of all molecules [number of molecules per unit time] to the process chamberxe2x80x9d.
In the past it was common practice to feed HCl gas to the oxidation furnace. Although this gas was effective for this application, its use has several drawbacks. Because of its corrosive nature, this gas deteriorates the metal distribution lines as well as the metal components in the gas management system. Such corrosion phenomena result in highly undesirable metallic contamination of the gases. Moreover the handling of the pressurized gas cylinders requires special care.
Because of these drawbacks the industry has used, 1,1,1-trichloroethane (TCA) as the source for Cl in the furnace. TCA is a volatile liquid and can be introduced into process tools via Teflon(trademark) tubing thereby avoiding the corrosion phenomena faced with HCl. Since TCA has been identified as an ozone depleting substance, attacking the stratospheric ozone layer, its production, use and/or transportation has been restricted or even banned.
In response the industry has come up with ozonelayer-friendly replacement substances for TCA such as trans-1,2-dichloroethylene (DCE) and oxalyl chloride (OC). The replacement with DCE is the subject of the United States patent U.S. Pat. No. 5,288,662. The replacement with OC is the subject of the European Patent EP 0 577262 B1. In virtually all industrial practice of Cl-oxidation, a Cl-equivalent concentration of the Cl-source of 1-3% is used as illustrated by the example1 and comparison2of the European Patent EP 0 577262 B1 and in the United States patent U.S. Pat. No. 5,288,662. In general, when Cl-carbon precursors are used, care has to be taken to get a complete enough combustion of the molecule. Regarding said combustion, the chemistry for OC is substantially different from that for either one of TCA or DCE. Because OC contains no hydrogen, all the Cl in the precursor is made available in the process tube as Cl2 (equation 1), provided of course that water is not deliberately added. In contrast, as in HCl itself, in the TCA and DCE molecules the number of hydrogen atoms equals the number of chlorine atoms. Therefore, during combustion, TCA (equation 2) and DCE (equation 3) are sources for HCl. Only a fraction, typically about 10%, of the so formed hydrogen chloride is (further) oxidized to form Cl2 and H2O, according to the equilibrium of the reaction (equation 4). It is obvious that said fraction depends on the parameters which affect the thermodynamical equilibrium like the percentage O2 in the ambient. When this percentage is about 100%, said fraction is about 10%.
C2Cl2O2+O2 xe2x86x92 Cl2+2 CO2xe2x80x83xe2x80x83(1) 
C2H3Cl3+2 O2 xe2x86x92 3 HCl+2 CO2xe2x80x83xe2x80x83(2) 
C2H2Cl2+2 O2 xe2x86x92 2 HCl+2 CO2xe2x80x83xe2x80x83(3) 
4 HCl+O2 xe2x80x832 Cl2+2 H2O xe2x80x83xe2x80x83(4) 
Consequently to ensure a complete combustion of TCA and DCE, the O2 concentration should be very high (a multiple e.g 10-fold of the stoichiometrical requirement) and the temperature should be sufficiently high. Therefore in the state of the art applications of Cl-carbon precursors, the Cl-source is only on when the larger fraction of the process chamber ambient consists of O2. Typically almost pure O2 is used and only a smaller fraction of N2 is added through the introduction of the Cl-carbon using a bubbler, as illustrated by the United States patent U.S. Pat. No. 5,288,662.
The ongoing downscaling of CMOS device dimensions, in particular the gate length, demands for an ongoing reduction of the gate oxide thickness in order to meet the required device performance specifications. With this required shrinkage of the thickness of high quality gate oxides, the use of organic molecules to introduce Cl in the furnace has become more critical. In order to obtain a good thickness control the process for growing thin oxides requires a milder overall oxidation condition, especially a low temperature treatment and a reduced oxygen concentration. Consequently, the organic Cl containing molecules will undergo also a milder oxidation, yielding the risc of only partial combustion of said molecules and risc of formation of highly toxic compounds like e.g. phosgene.
In recent years a new process was introduced referred to as the xe2x80x9cpyro-cleanxe2x80x9d, see B.-Y. Nguyen et al, in Tech. Dig. 1993 Symp. on VLSI Technol., (JSAP, Tokyo, 1993) p. 109. In this process an in-situ low temperature Cl-treatment prior to the gate oxidation process is used. The motivation for this process is based on the fact that the diffusion constant and the solubility of a number of metals in silicon increases strongly with increasing temperature. The purpose of this process is to remove metallic contamination before the onset of diffusion of metal into bulk silicon. Typically a 30 minutes treatment at 650xc2x0 C. is performed using an inert (e.g. N2) ambient containing O2 at a volume concentration of 2%. As a Cl source, HCl was chosen with a Cl-equivalent concentration of 3%. The addition of the small amount of oxygen is expected to be beneficial with regard to organic contamination, preventing destabilisation of the SiO2 phase and limiting surface etching and roughening. At the same time the oxygen concentration should be kept low enough in order to limit the thickness of the oxide layer grown during this pre-oxidation step, particularly when the final oxide layers that are to be grown should be thin. The process conditions for the xe2x80x9cpyro-cleanxe2x80x9d in B.-Y. Nguyen et al, in Tech. Dig. 1993 Symp. on VLSI Technol., (JSAP, Tokyo, 1993) p. 109 are a low temperature, a Cl-equivalent Cl-source concentration of 3% and a low oxygen concentration. In cited document HCl is used as a chlorine source. When using a chlorine-carbon source, it is obvious that this can result in a partial combustion, which is undesirable. Some early attempts in performing this process using TCA or DCE have even resulted in deposition of soot on the furnace wall and on the wafer surfaces, which is not possible when using OC. Moreover, even in an ambient with a high percentage of oxygen (even up to 100%) it is common knowledge that each of these substances have a minimum oxidation temperature below which a complete combustion is not possible. This minimum temperature is 800xc2x0 C. for TCA, about 700xc2x0 C. for DCE and as low as 400xc2x0 C. for OC. As mentioned above, the use of the corrosive gas, HCl, in this process leads to potential danger of corrosion of the gas distribution system and is therefore undesirable.
In an attempt to avoid the use of a corrosive gas and to overcome partial combustion of a Cl-carbon precursor, another approach, making use of a separate burn-box, has been proposed e.g. by Damon DeBusk et al, Miocro Sept. 1995, p.39. In this additional burn-box the organic precursor can be oxidized before being introduced into the process chamber holding the wafers to be oxidized. The relatively higher O2 concentration, the potentially higher temperature and a long residence time of the gases in this burn box, contribute to obtain a better combustion. But this process has some drawbacks. The bum-box solution requires additional hardware and/or software (a torch-like device and modification in control hard and/or software). The use of a burn-box for the gases to be burned first and particularly the increased residence time in the burn-box may limit the control over the reactor ambient, particularly with respect to switching of the ambient. This in turn can result in a degradation of process control. And finally the burn-box approach may only be marginal in solving the problem. Particularly in case of processes with a low oxygen concentration. To illustrate this an example making use of DCE is taken where a Cl-equivalent concentration of 3% is aimed. To convert DCE to HCl and CO2 stoichiometrically requires 2 oxygen atoms per Cl atom. To convert DCE to Cl2 and CO2stoichiometrically requires 2.5 oxygen atoms per Cl atom. In case of full combustion of DCE, i.e. all C converted to CO2, typically a mixture of a larger fraction of HCl and a smaller fraction of Cl2 is formed. Given said 3% Cl-equivalent concentration, an O2, volume concentration of 3 to 3.5% is required just to stoichiometrically match for the combustion of DCE. A large O2 excess, in the order of 10 times is typically required to obtain the same amount of Cl2 and to obtain full combustion during the finite residence time. Consequently the overall O2 volume-concentration in the process chamber will be above 30%. This is clearly a much too high value to match the xe2x80x9cpyro-cleanxe2x80x9d process. This severally limits the application of this technique to processes with relatively high oxygen concentration.
The present invention provides a solution to overcome these drawbacks.
It is the aim of the present invention to provide an efficient low oxidation power process that allows the growth of ultra-thin gate-oxides in a conventional furnace used in a standard CMOS IC-processing environment by using chlorine. Said process should avoid corrosion by using a Cl-carbon precursor and should at least maintain a performance equivalent to current HCl based processes.
In a first aspect of the invention a method is disclosed of in situ cleaning a silicon substrate by performing at least one heating step in a gas phase ambient comprising Cl2 and preferably a very low concentration of oxygen, typically a volume concentration between 2% and 5% or below. This in situ Cl-clean can take place in a conventional oxidation furnace. Preferably said Cl-clean is performed in a gas phase ambient comprising the reaction products of oxygen (O2) and an organic Cl-carbon based substance, preferably oxalyl chloride. Said reaction products can comprise oxygen, Cl-atoms and Cl2. The aim of this in situ Cl-clean is to remove the metal surface contaminants before they can diffuse into the substrate.
In a second aspect of the invention a method is disclosed for growing a thin silicon oxide, preferably SiO2, on a silicon substrate using a gas phase ambient comprising Cl2. Said growth of silicon oxide can take place in a conventional oxidation furnace. More specifically a method is disclosed of growing said thin silicon oxide on said silicon substrate using a gas phase ambient comprising the reaction products of oxygen (O2) and an organic Cl-carbon based substance, preferably oxalyl chloride. Said reaction products can comprise oxygen, Cl-atoms and Cl2. Using the method of the invention a controlled growth of thin silicon oxide layer on a silicon substrate can be achieved with the thickness of said silicon oxide layer in the range of 0.1 to 1 nm or 1 to 8 nm or above 8 nm.
In a third aspect of the invention, a method is disclosed for growing a thin silicon oxide on a silicon substrate comprising at least two steps, in one step in an situ Cl-clean is performed and in another step a thin silicon oxide layer is grown on the silicon substrate using a gas phase ambient comprising the reaction products of oxygen and an organic Cl-carbon based substance, preferably oxalyl chloride. Using this method a controlled growth of a high quality thin silicon oxide layer on a silicon substrate can be achieved with the thickness of said silicon oxide layer in the range from 0.1 to 1 nm or 1 to 8 nm or above 8 nm.
Said substrates of said first, second and third aspect of the invention are kept at a temperature of 900xc2x0 C. or below. The present invention includes temperature ranges from 500xc2x0 C. to 550xc2x0 C., from 550xc2x0 C. to below 700xc2x0 C. and from 700xc2x0 C. to 900xc2x0 C. Preferably a temperature of 650xc2x0 C. is used. Heating steps are typical in the range up to 30 minutes or 80 minutes or higher, but the invention is not limited thereto. In particular even oxidation or anneal times in the order of seconds may be used but such short anneal times usually require the use of pre-burning box. Furthermore a low Cl-equivalent concentration of oxalyl chloride in the gas phase ambient is used. Said Cl-equivalent concentration of oxalyl chloride can be in this range of about 0.001-0.3%. Said Cl-equivalent concentration of oxalyl chloride can also be in the range of 0.3-0.5% or in the range of about 0.5-1%. Higher concentrations can also be used. The gas phase ambient can also further comprise other gases or compounds that do not influence the efficiency of the method or that do not introduce contaminants in the grown silicone oxide. The gas phase ambient that is used can further comprise hydrogen or water steam but this reduces the efficiency.
In a further aspect of the invention a method of growing a silicon oxide layer on a silicone substrate by means of a thermal oxidation in a furnace is disclosed, comprising the steps of:
heating said substrate in said furnace, preferably said heating is executed in at least one step to at least one temperature typically in the range from 500xc2x0 C. to 1000xc2x0 C.;
flowing a gaseous mixture into said furnace, said mixture comprising oxygen and Cl2, said Cl2 being generated by an organic chlorine-carbon source, while keeping said furnace at a temperature below 700xc2x0 C.;
holding said silicon substrate in said furnace until said silicon oxide layer on said substrate is formed.
In still a further aspect of the invention a method of growing a silicon oxide layer on a silicon substrate by means of a thermal oxidation in a furnace is disclosed, comprising the steps of:
heating said substrate in said furnace;
flowing a gaseous mixture into said furnace while keeping said furnace at a temperature below 900xc2x0 C., said mixture comprising oxygen and Cl2, said oxygen having a volume concentration of 5% and below, said Cl2 being generated by an organic chlorine-carbon source;
holding said silicon substrate in said furnace until said silicon oxide layer on said substrate is formed.